A cylinder head gasket is typically provided between the cylinder block and the cylinder head of an internal combustion engine to prevent leakage of combustion gas, coolant, and oil. Various types of such gaskets have been used. For example, Japanese Laid-Open Patent Publication No. 9-126322 discloses such a gasket. The gasket disclosed in the publication includes a gasket body (plate) having water holes and oil holes, and metal grommets each provided in a position corresponding to one of the water holes and the oil holes. Further, a coating layer made of rubber or resin is formed on each of the surfaces of the grommets contacting the cylinder head and the cylinder block, and on each of the surfaces of the grommets contacting the gasket body. The coating layers prevent the metal components (the cylinder head and the grommets, the cylinder block and the grommets, and the grommets and the gasket body) from directly contacting each other.
Besides the above described publication, prior art documents related to the present invention include Japanese Laid-Open Patent Publications No. 2003-322256 and No. 2003-56710.
A typical cylinder head gasket is used in harsh environments, receiving a great compression load from a cylinder head and a cylinder block as head bolts are tightened or heat of combustion gas generated as an engine operates. In addition, if grommets having coating layers are made of a metal that has a different coefficient of thermal expansion from those of the cylinder head and the cylinder block, the amount of thermal expansion of the grommets is different from that of the cylinder head and the cylinder block. Compression load, heat of combustion gas, and a difference of the amount of the thermal expansions can cause the coating layers to come off the grommets. If the coating layers come off, the metal components can directly contact each other. When different types of metal contact each other, galvanic corrosion (electrolytic corrosion) can occur due to the potential difference at the contacting sections depending on the external environment.